1. Field of the Invention
The present invention generally relates to a heat roller fixing arrangement for use in an electrophotographic copying apparatus, and more particularly, to a heat roller fixing device which includes a heating roller formed by laminating a fluorine resin or fluorine plastic layer on an electrically conductive core member through a primer layer, and a pressure roller constituted by forming an electrically insulative layer on another electrically conductive core member for passing therebetween a copy paper sheet which carries a positively charged toner image formed thereon by the known electrophotographic method, so as to fix the toner image onto the copy paper sheet, and especially, to a heat roller fixing device of a type not applied with an offset prevention agent on the surface of the heating roller.
2. Description of the Prior Art
The heat roller fixing device of the above described type has such advantages that copy material, for example, in the form of a copy paper sheet is provided with a favorable hand feeling after the fixing without soiling thereof by an offset prevention agent, and that, since no means is required for applying the offset prevention agent, the fixing device may be simplified in construction, with a consequent reduction in cost, as compared with heat roller fixing devices which employ the offset prevention agent. On the contrary, however, the heat roller fixing device of the above described type without employing the offset prevention agent has a drawback in that the undesirable offset phenomenon tends to take place very easily. Conventionally, various means have been proposed for preventing the offset phenomenon, for example, by employing a heating roller coated with a fluorine resin layer superior in release properties and heat resistance, as an offset prevention layer for the improvement of releasing between the heating roller and toner, or by proper selection of binder resins for the toner or addition of release agents such as low-molecular weight polypropylene and the like for the improvement of release properties of the toner itself. However, in the fixing devices in which the toner is positively charged, it has been difficult to perfectly prevent the offset phenomenon unless the offset prevention agent is employed.
As a result of various studies and investigations carried out for solving the problems in the conventional arrangements as described above, the present inventors have found that, in the fixing device in which a heating roller constituted by forming a fluorine resin layer on an electrically conductive core member directly or through a primer layer is combined with a pressure roller constituted by forming an electrically insulative layer on another electrically conductive core member, the offset phenomenon takes place only during the initial period of fixing, with the generation thereof being suspended after lapse of a certain period of time, due to such causes that both the heating roller and pressure roller are electrically charged by friction or triboelectrical contact between said rollers or between the respective rollers and copy paper sheets, with the heating roller being started to be negatively charged at the initial stage so as to be positively charged, on the contrary, after lapse of a certain period of time, and that the time period in which the offset phenomenon takes place approximately corresponds to the time period during which the heating roller is negatively charged, and for example, when the device is operated at a fixing speed of 12 sheets/minute, the offset phenomenon takes place for about 10 minutes, during which period, properly copied items can not be obtained. The present inventors have also discovered that, for preventing the negative charging of the heating roller at the initial stage of operation of the fixing device, carbon black may be added to the primer layer, and completed the present invention based on the findings as described in the foregoing.